Process for preparing hydrogen-nitrogen mixtures from coke oven gas



Jan. 21, 1958 P. J. HARINGHUIZEN 2,82

PROCESS FOR PREPARING HYDROGEN-NITROGEN MIXTURES FROM COKE OVEN GASFiled April 6, 1956 2 Sheets-Sheet 1 1 FBGE 2 QBQ 7a 2 Jan. 21, 1958 P.J. HARINGHUIZEN 2,820,759

PROCESS FOR PREPARING HYDROGEN-NITROGEN MIXTURES FROM COKE OVEN GASFiled April 6, 1956 2 Sheets-Sheet 2 F363 F'iGA- Sa J i United StatesPRQCESS FOR PREPARING HYDROGEN-NITRG- GEN MIXTURES FROM COKE OVEN GASlieter J. Haringhuizen, Geleen, Netherlands, assignor to Stamicarbon N.V., Heerlen, Netherlands Application April 6, 1956, Serial No. 576,648Ilaims priority, application Great Britain April 7, 1955 10 Claims. (Cl.252-374) The present invention relates to the preparation of a mixtureof hydrogen and nitrogen suitable for the synesis of NH using coke ovengas as the source of hydrogen.

A gaseous residue consisting essentially of hydrogen can be obtainedfrom coke oven gas by cooling the gas under pressure, e. g. 10 to 30atmospheres and 165 to -190 C., to condense therefrom less volatilecomponents. The resulting gaseous residue contains, in ad dition tohydrogen, about 4% by volume of nitrogen, about 4% by volume of CO and,dependent upon the cooling, from 2 to 8% by volume of CH The presentinvention is particularly concerned with novel improvements for washingthis impure gaseous residue with liquid nitrogen so as to remove CH andCO substantially quantitatively.

It is customary for the washing of the gaseous residue to be carried outby flowing the residue through a washing column, while the residue ismaintained under the pressure utilized in separating the less volatilecomponents therefrom, in counter-current relationship to liquid washingnitrogen. In order to eifect a high degree of CO removal, thetemperature in the washing column must be very low although it may bevaried within relatively wide limits depending upon the pressureutilized. As a specific illustration, a column temperature of about l90C., can be utilized when the separating pressure is 13 atmospheres.

In order that the low temperature necessary for efiecting asubstantially complete removal of the C may be maintained, it iscustomary to install a condenser, either in front, or on top, of thewashing column, through which the gaseous residue is passed and cooledby the evaporation therein of liquid nitrogen under a constant pressure.Thus, if the separation of the coke oven gas is carried out at the usualpressure of about 13 atmospheres, the nitrogen in the condenser isevaporated at a pressure slightly higher than 1 atmosphere, namely, 1.3atmosphe es. As a result, the nitrogen in the condenser has atemperature of -l93 C., and the gas mixture flowing through thecondenser is cooled to a temperature which is slightly higher, namely,--190 C.

Both positions of the condenser, i. e. in front or on top, of thecolumn, involve certain advantages and drawbacks. Thus, if the condenseris placed in front of the Washing column, the gaseous residue from thecoke oven gas separation enters the washing column at a temperature ofabout -l90- C. Nearly immediately thereafter the temperature drops a fewdegrees owing to rapid evaporation of part of the washing nitrogen whichgoes into the gas mixture and takes up the volume of the methane, thelatter being completely condensed at this low temperature. As a result,the temperature in the washing column is always lower than -190 C., sothat it is possible to remove the CO nearly quantitatively to, e. g. X10percent by volume by washing with relatively small amounts; ofliquid'nitrogen.

The removal ofCO depends not-only. on the tempera- 2,820,769 PatentedJan. 21', 1958 nitrogen rising in the gaseous state V is:

56 cu. m. at 195 102 cu. m. at -l cu. m. at l85 258 cu. m. at -182 Inorder to obtain the most economical consumption of liquid washingnitrogen possible the washing process is preferably carried out in sucha way that L is equal to V or even slightly smaller (e. g. 5-10 cu. in.)than V. Expressed difierently, the process is carried out under suchconditions that the amount of washing nitrogen introduced into thewashing column evaporates completely in the column and is removed at thetop thereof together with the hydrogen freed of methane and CO, whilethe amount of liquid nitrogen collected at the bottom of the, column,which is mixed with a large amount of CO, is equal to or slightlysmaller than the amount of nitrogen continuously introduced into thewashing columnas part of the gaseous residue from the coke oven gasseparation (about 4% by volume N If the condenser is placed in front ofthe column, the temperature in the column is very low. As a result, Vhas a relatively low value and L may also be small. Due to therelatively small values for L and V, the degree to which the CO isremoved is highly dependent on flue-- tuations in the amount L and,moreover, the CO percentage of the washed hydrogen'immediately rises toan undesired level when the amount of L becomes too small.

Consequently, the advantage of installing the condenser.

in front of the washing column is that a constant and very lowtemperature prevails in the column with the result that the removal ofCO is very effective if the correct amount of washing nitrogen issupplied. On the other hand, there is the disadvantage that the washingtreatment requires great attention from the operating personnel since asmall decrease in L, i. e. a decrease in the amount of liquid washingnitrogen supplied at the top of the column, immediately allows CO topass out of the column admixed with the hydrogen.

If the condenser is placed on top of the column, the

temperature in the washing column depends to a large of the column and,as a result, the amount of gaseous liquid V flowing upwards ismuchgreater. In order to obtain the lowest possible loss of nitrogen tothe residual gas (the fraction which is collected and discharged at thebottom of the column in the liquid state), the amount of liquid nitrogenL flowing down in the wash ing column, which in this case is equal tothe sum of the liquid nitrogen supplied at the top of the washing columnand the condensate formed in the condenser, is chosen to be equal to V,or preferably to be evenslightly smaller than V. Consequently, the valueof L is much greater thanin the casewhere'. the condenser is arranged infront of the column.

Due to the higher values of L and V when the condenser is placed. on topof the column, the value of the reflux ratio L/ V is much less sensitiveto a decrease in L when compared with what would be the case at the sameabolute dropin L if :the condener were placed in front of the column. Infact, ithas been observed in practice that tr washing column with thecondenser placed on top requires less attention from the operatingstaff, as regards the feed of liquid washing nitrogen, than a washingcolumnwith the condenser placed in from. As a result of this, a negativeL-V can be safely maintained in the washingprocess so that the processcarried out with a condenser placed on: top of the washing column isextremely economical insofar as loss'of nitrogen-to the fraction ofresidual gas is concerned. On the, other hand, there is the disadvantagethat, as a result ofthehigher temperature in the washing column, theremovalof CO is less thorough than in a washing column with the'condenser arranged in w front. Additionally, the washingcolumn isparticularly sensitive to temperature since'there is no way ofcorrecting the incoming temperature for the gaseous residue from thecoke oven gas separation, for example, by means of a condenser in whichliquid nitrogen is evaporated under a constant pressure. Consequently, arise in the temperature of the gaseous mixture to be washed immediatelyresults in a strong rise in the CO content of the washed gas.

It has now. been found that if the condenser, in which the coolingis'effected by evaporation of nitrogen under a constant pressure, isplaced not on top or in front of the column, as previously proposed, butwithin it, or in such a way that two washing compartments are formed,which are separated by the condenser, the drawbacks of theabove-mentioned washing systems are avoided and a hydrogen-nitrogenmixture of a very low CO content as well as a very reliable apparatuswhich requires littlesupervision are obtained. The loss of nitrogen tothe residual gas fraction is very small and the CO removal isinsensitive to temperature fluctuations in the arriving gaseous residueof the coke oven separation which is to be washed.

As regards the removal of CO by means of liquid nitrogen, it has alreadybeen proposed (see Dutch patent specification No. 74,830; French patentspecification No. 861,175; U. S. patent specification No. 2,293,601) touse a washing column consisting of two washing compartments separatedbya condenser .and to feed as coolant into this condenser, the hydrogenunder pressure and saturated. with nitrogen coming from the top of thewashing column, liquid nitrogen having been addedto this hydrogen,before it enters the condenser. This liquid nitrogen must then evaporateunder partial pressure to cool the gas mixture flowing through thecondenser and rising in the washing column. However, in actual practice,the dificrence in temperature between the coolant and the gas stream tobe cooled proves to be practically nil, so that liquid nitrogen is notevaporated and no cooling effect is obtained.

In contradistinction to the foregoing, the processof the presentinvention employs boiling liquid nitrogen as cool ant in the condenser,the nitrogen being kept under a constant pressure preferably slightlyhigher than atmospheric, e. g. 1.05 to 1.60 atmospheres. In this way, aconstant and very low temperature is maintained in the condenser.

The present invention is more fully explained by the attached drawingwherein Figures 1 and 2 represent prior art constructions and Figures 3and 4 illustrate variations of the process and apparatus of theinvention.

According to Figure 1, the cold gaseous residue of the coke oven gasseparation is conducted to condenser 2 by means of conduit 4. Theresidue is cooled indirectly in the condenser by the evaporation ofnitrogen supplied by the apparatus 3 wherein nitrogen is compressed toabout 200 atmospheres and strongly cooled. The high-pressure rntrogenflows from apparatus 3 through conduit 7 and into the condenser afterexpansion thereof in valve 7a. The evaporated nitrogen is returnedthrough conduit 8 into apparatus 3. 1 V

which is further-cooled in condenser-2, flows through conduit 10 andinto the bottom part of washing column 1. Liquid nitrogen is supplied tothe top of the washing column through conduit 6 and valve 6a. Part ofthis nitrogen evaporates in the column while the remainder thereof ismixed with condensed methane and CO, discharged through conduit 11 andvalve 11a, and evaporatedby heat exchange with incoming coke oven gas asis the condensate formed in condenser 2 whichis discharged through conduit 9 and valve 9m The hydrogen-nitrogen mixture freed of CO is discharged fromthe washing column through conduit 5 and, after heatexchange with arriving coke oven gas and the furtheraddition' ofnitrogen to form a mixture of Hg and N in the volume'ratio -3 :1, isintroduced into the N11,, synthesis process.

In Figures 2-4, the parts corresponding to those in Figure 1 aredenot'edby means of the same reference marks as used in thelatterfigure. In theapparatus of Figure 2, the condenser is placed on top of the column, sothat the condensate formed in the condenser fiows into the top part ofthe washing column.

The cold residue of the coke oven gas separation flows directly throughconduit 4 into the bottom section of washing column 1, in which it iswashed by means of liquid nitrogen supplied through conduit 6. Thewashed gas flows through condenser 2, in which indirect cooling takesplace by nitrogen evaporating in the condenser.

Figure 3 exemplifies. the process and apparatus of the presentinvention. As shown therein, the cold gaseous residue of the coke ovengas separation first, flows into the bottom section of a washingcompartment 1a and subsequently through condenser 2 into washingcompartment 1b. Liquid nitrogen supplied through conduit 6 is fed intothe top section of washing compartment 1b.

The washing column may also comprise two separate parts, as'is shown inFigure 4, which communicate by means of a U-shaped conduit 12, throughwhich the wash ing liquid flows from one washing compartment into theother and by means of a conduit 13, which conducts gas. This latterembodiment offers the advantage that damming of the liquid in thecondenser by arriving gas is impossible. V

In the process of the invention, the combined height of the two washingcompartments is not greater than the height of the washing columncommonly used in the conventional construction with only one washingcompartment. Furthermore, it has been found that the first washingcompartment, located under. the condenser, may be smaller than thesecond washing compartment. Thus, if the number of theoretical plates inthe first washing compartment is about A; to Mt of the totalnumber oftheoretical plates needed, the CO removal reaches its optimum value,under otherwise equal conditions.

The following table illustrates various CO contents for the washed gas,obtained when using apparatus according to Figures 1, 2 and 3 or 4 atdifierent values of L-V.

The CO content relatesto the eventual NH -synthesis mixture (3 parts byvolume of H and 1 part by volume of N obtained by making up the shortageof nitrogen after the washing treatment. The data given is based onpassing at a pressure of 13 .atm. the gaseous residue of 1000 cu. m. N.P. T..of coke oven gas, i. e. 620 cu. m. H 39.7 cu. m. N 41.8 cu. m. COand 59.5 cu. m. CH, per. hour through the apparatus. In all three cases,the temperature of the boiling nitrogen in the condenser was 193 C., sothat the cooled gas mixture leavingthe condenser had a temperature of C.Accordingly, the temperature in the bottom section of the washing columnaccording to Figure l was 195 C., while the temperature of'the' gasprevailing throughout the column of Figure 2 and in the lower washingcompartment of Figure 3 was --'182 113. In the second or upper washingsteam-res compartmentof Figure 3', the temperature was --1'90 C; Thetotali number of theoretical plates was-equal in: all threecases andamounted to 22.

From the above CO contents,,it. can be. seen that the system accordingto the invention is considerably superior to the washing systems.employed hitherto, particularly for negative values of L-V (whichrepresents a very economical consumption of washing nitrogen).Furthermore, it is apparent that, when the process according to theinvention is used, the removal of CO is far less sensitive to a decreasein the amount of washing nitrogen, which results in a fall of L-V, thanwhen conventional systems are used.

It will be appreciated from the foregoing that the process describedherein represents a novel improvement in the preparation ofhydrogen-nitrogen mixtures from coke oven gas wherein the gas is cooledunder pressure to condense low volatile constituents and the resultingcold gaseous residue consisting primarily of hydrogen and small amountsof nitrogen, carbon monoxide and methane is subsequently washed underpressure with liquid nitro gen. Broadly stated, the improved process ofthe invention comprises initially washing the cold gaseous residue bypassing same upwardly through a first washing zone wherein the residueis washed by counter-current contact with liquid nitrogen, then passingthe thus treated gaseous residue upwardly through an intermediatecondensing zone wherein the residue is indirectly cooled with boilingnitrogen under a constant pressure preferably slightly higher thanatmospheric and a condensate of nitrogen is obtained from the residue,thereafter further washing the gaseous residue by passing same upwardlythrough a second washing zone in counter-current contact with liquidnitrogen, collecting liquid washing nitrogen contaminated with carbonmonoxide from said second washing zone and utilizing the collectednitrogen and the condensate from the condensing zone as the liquidnitrogen for washing the gaseous residue in the first washing zone.

According to the present process, the nitrogen utilized for washing inthe second washing zone is substantially pure nitrogen. Preferably, thetemperature of the gas being washed in the first and second washingzones is about l82 C. and 190 C., respectively, although othertemperatures include 175 to -l88 C. in the first zone and 185 to 195 C.in the second zone. The temperature of the gas is, therefore, reduced byabout 3 to C. in the condensing zone. The pressure of the gas during thetreatment is preferably the same as that used in initially condensinglow volatile constituents from the coke oven gas, e. g. 10 to 30atmospheres with 13 atmospheres particularly advantageous.

As shown in Figures 3 and 4, the apparatus of the invention, broadlydescribed, comprises an upper gas washing compartment, means forsupplying liquid washing nitrogen downwardly through the compartment incountor-current relationship to gas flowing upwardly therethrough, meansfor collecting liquid washing nitrogen from the upper washingcompartment, a lower washing compartment, means for passing gas to bewashed upwardly and successively through the first and second washingcompartments, condensing means for indirectly cooling said gas betweenthe first and second washing compartments, the cooling means comprisingboiling nitrogen under a constant pressurepreferably slightly aboveatmospheric, means for collecting condensate ob-.- tained from the gasin said condensing meansand means for supplying liquid nitrogencollected from the upper compartment and the condensate from thecondensing means to the lower compartment as the wash liquidinthelastrrnentioned compartment.

It will be appreciatedthat various modifications may be made in theinvention describedherein. Hence; the scope of the invention is definedin: the' appended claims wherein:

I claim:

1. In a process for preparing hydrogen-nitrogen mixtures from coke ovengas by cooling said gas; under pressure to condense low volatileconstituents and there? after washing the resulting cold gaseousresidueunder pressure by counter-currently contacting the. same'withliquid nitrogen to remove methane and carbon monoxidetherefrom, theimprovement which comprises initiallywashingsaid cold gaseous residue bypassing same upwardly through a first washing zone wherein saidresidueis washed by counter-current contact with liquid nitrogen, thenpassing the thus treated gaseous residue'upwardly through anintermediate condensing zone wherein said residue is indirectly cooledwith boilingnitrogen under aconstant pressure preferably slightly higherthan atmos pheric and a condensate of nitrogen is obtained from saidresidue, thereafter further washing said gaseous residue by passing sameupwardly through a second washing zone in counter-current contact withliquid nitrogen, the height of said first washing zone beingsubstantially less than that of said second washing zone, collectingliquid washing nitrogen contaminated with carbon monoxide from saidsecond washing zone and utilizing the collected nitrogen and thecondensate from said condensing zone as the liquid nitrogen for washingsaid gaseous residue in said first washing zone.

2. The process of claim 1 wherein the liquid nitrogen utilized forwashing in said second washing zone is substantially pure nitrogen.

3. The process of claim 1 wherein said first and second washing zonesare superimposed and the height of said first washing zone comprisesfrom A: to A of the joint height of said first and second washing zones.

4. The process of claim 1 wherein the pressure of the boiling nitrogenin said condenser is between 1.05 and 1.60 atmospheres.

5. The process of claim 1 wherein said gaseous residue is maintained ata pressure of 10 to 30 atmospheres during said washing and intermediatecooling steps.

6. The process of claim 1 wherein the pressure of the boiling nitrogenin said condenser is 1.3 atmospheres and the pressure of said gaseousresidue during said washing and intermediate cooling is maintained at 13atmospheres.

7. The process of claim 1 wherein the temperature of said gaseousresidue is between and l88 C. in said first Washing zone and saidresidue is cooled about 3 to 10 C. in said intermediate condensing zone.

8. The process of claim 1 wherein the gaseous residue passed into saidfirst washing zone consists essentially of, by volume, about 4%nitrogen, about 4% carbon monoxide, from 2 to 8% methane and theremainder hydrogen.

9. In a process for preparing hydrogen-nitrogen mixtures from coke ovengas by cooling said gas to a temperature of about C. under about 13atmospheres pressure to condense low volatile constituents therefrom andobtain a cold gaseous residue consisting essentially of, by volume,about 4% nitrogen, about 4% carbon monoxide, from 2 to 8% methane andthe remainder hydrogen, and thereafter washing said cold gaseous residueat said pressure of about 13 atmospheres by countercurrently contactingsame with liquid nitrogen to remove carbon monoxide and methane, theimprovement whereby carbon monoxide and methane are substantiallyquantitaafter further washing said cooled gaseous residue at about ,7

tl90f-,,C;-by passing same'upwardly through a second tialiy ;less thanthat of said ;second washing zone, ;withdrawinga; gaseous mixture,consisting essentially ofhydrogen aandrnitrogen from said second,washing zone,

collecting the liquid washing nitrogen from said second washing zone,utilizing this collected nitrogen and the condensate -fro m saidcondensing zone as the liquid nitrogenifor washing said gaseous residuein said first washing zone. 1

10.-Apparatus for preparing gaseous hydrogen-nitrogen mixtures from cokeoven gas comprising an upper gas washing compartment, means forsupplying liquid l washing zone in counter-current contact withliquidnitro- I gen thegheight-of; said first washing zone being substan:

lastmentioned compartment;

$38: washingnitrogen downwardly through said compartment incounter-current relationship to gas flowing upwardly therethrough, meansfor collecting liquid ,washingnitro:

s from said pp as c mpart t. lpweruas -l ing compartment, the height ofsaid lower washing compartment being substantially less than that ofsaid upper gas washing compartment, means for passing gas to be washedupwardly and successively through said first and second washingcompartment, condensing means for indirectly cooling said gas betweenthe first and second washing compartments with boiling nitrogen under aconstant pressure, meansifor collectingv condensate-, oba -1tained fromsaid gas in said condensing means and means" for supplying liquidnitrogen collected from saidIjupp er' compartment and the condensate.fromvsaid condensing means to said lower compartment as the washliquidinReferences Cited in the file of this patent UNITED STATES PATENTS2,293,601. Etienne Aug. 18, 1942 2,692,484 Etienne Oct. 26, 19542,729,954 Etienne Jan. 10, 1956 2,743,590 Grunberg May 1, 1956

1. IN A PROCESS FOR PREPARING HYDROGEN-NITROGEN MIXTURES FROM COKE OVENGAS BY COOLING SAID GAS UNDER PRESSURE TO CONDENSE LOW VOLATILECONSITUENTS AND THEREAFTER WASHING THE RESULTING COLD GASEOUS RESIDUEUNDER PRESSURE BY COUNTER-CURRENTLY CONTACTING THE SAME WITH LIQUIDNITROGEN TO REMOVE METHANE AND CARBON MONOXIDE THEREFROM, THEIMPROVEMENT WHICH COMPRISES INITIALLY WASHING SAID COLD GASEOUS RESIDUEBY PASSING SAME UPWARDLY THROUGH A FIRST WASHING ZONE WHEREIN SAIDRESIDUE IS WASHED BY COUNTER-CURRENT CONTACT WITH LIQUID NITROGEN, THENPASSING THE THUS THREADED GASEOUS RESIDUE UPWARDLY THROUGH ANINTERMEDIATE CONDENSING ZONE WHEREIN SAID RESIDUE IS INDIRECTLY COOLEDWITH BOILING NITROGEN UNDER A CONSTANT PRESSURE PREFERABLY SLIGHTLYHIGHER THAN ATMOSPHERIC AND A CONDENSATE OF NITROGEN IS OBTAINED FROMSAID RESIDUE, THEREAFTER FURTHER WASHING SAID GASEOUS RESIDUE BY PASSINGSAME UPWARDLY THROUGH A SECOND WASHING ZONE IN COUNTER-CURRENT CONTACTWITH LIQUID NITROGEN, THE HEIGHT OF SAID FIRST WASHING ZONE BEINGSUBSTANTIALLY LESS THAN THAT OF SAID SECOND WASHING ZONE, COLLECTINGLIQUID WASHING NITROGEN CONTAMINATED WITH CARBON MONOXIDE FROM SAIDSECOND WASHING ZONE AND UTILIZING THE COOLED NITROGEN AND THE CONDENSATEFROM SAID CONDENSING ZONE AS THE LIQUID NITROGEN FOR WASHING SAIDGASEOUS RESIDUE IN SAID FIRST WASHING ZONE.